When filaments of material, such as metal wire or strip, are dip coated, for instance in molten zinc, aluminum or their alloys, it is normally necessary to strip excess coating material from the surface of the filament. There are a number of known ways of achieving this, one of which is generally called gas jet wiping. In gas jet wiping processes a stream of a gas is caused to impinge upon the filament to strip the excess coating material therefrom. Typical jet wiping apparatus and nozzles therefore are described in the following patent specifications:
______________________________________ U.S. 2,194,565 3,060,889 3,270,364 3,611,986 3,707,400 3,736,174 4,287,238 Australian 458,892 537,944 539,396 544,277 ______________________________________
In coating filaments by the known gas jet wiping processes, and in particular in the coating of ferrous wire with molten metals such as zinc, aluminum or their alloys, a number of problems arise.
For planar material such as metal sheet, gas jet wiping has been effective in controlling the thickness of the coating metal on the material and in producing a smooth uniform surface finish. For angular filaments such as circular and non-circular wire, tubular material and narrow strip the geometry of the material being wiped presents problems not occurring with planar material. Metal oxide builds up on the filament beneath the wiping region and forms a ring or band around the complete perimeter of the filament. Periodically this build up of oxide becomes sufficient to burst through the wiping gas stream, because of the filament's small circumference, to form thick rings or bands of coating on the filament, which is undesirable. The present invention is directed towards overcoming this problem.
A number of prior art gas jet wiping processes have overcome this problem by enclosing the filament within a hood which provides a completely protective atmosphere to the filament between when it leaves the metal bath and when it is wiped, such as is outlined in U.S. Pat. Nos. 3,707,400 and 4,287,238.
A problem with the process disclosed in U.S. Pat. No. 3,707,400 is that it has been difficult or impossible to control the thickness of the coating metal on the filament by adjusting the quantity of gas entering the gas jet wiping nozzle. In order to alter the coating thickness without changing to a different sized nozzle, it has been necessary to alter the throughput speed of the filament directly proportional to the thickness of coating required, i.e. decreased coating thicknesses require decreased throughput speeds and increased coating thicknesses require increased throughput speeds. This requirement to adjust the throughput speed of the filament in order to obtain a desired coating thickness, is undesirable as it impedes the efficient operation of other sections of a galvanising line e.g. the heat treatment and cleaning sections and changes the quantity of wire produced.
A problem with the process disclosed in U.S. Pat. No. 4,287,238 is that splatterings of coating metal form on the surface of the nozzle's wire orifice, especially at higher wiping gas pressures and filament speeds. These splatterings, which have been removed from the filament as a consequence of the wiping action, are a problem, because they build up quickly on the surface of the nozzle's wire and gas orifices and eventually come into contact with the filament, interfere with the effective wiping action of the gas and cause surface imperfections on the filament. A further problem with this process is the relatively large quantities of gas consumed, which make it more economical to use alternative wiping processes such as pad wiping, where the filament is physically wiped by asbestos or similar material or the process as outlined in U.S. Pat. No. 3,892,894.
A still further problem with the process according to U.S. Pat. No. 4,287,238 is the relatively large overall dimensions of the wiping apparatus. Its overall size means that wires must be spaced further apart at the exit end of the hot dip metal bath than would otherwise be the case and as such, fewer wires can be processed, resulting in reduced production. A variation of this process, as outlined in Australian patent specification 539396, where the gas jet wiping is carried out without a protective hood, suffers from the problems described above in connection with the process of U.S. Pat. No. 4,287,238, and additionally with the problem of thick coating rings remaining on the filament after being wiped, also mentioned above. The present invention is directed towards overcoming the abovementioned deficiencies in known gas jet wiping processes and the apparatus used to carry this out.
U.S. Pat. No. 3,736,174 discloses a gas jet wiping nozzle having a plurality of gas streams which are caused to impinge upon each other prior to striking the filaments being wiped. This arrangement allows the angle of impingement of the gas on the filament to be varied. While parts of the nozzle bear a superficial resemblance to the nozzle according to this invention, the nozzle according to this specification, when taken as a whole, does not show the physical configuration which produces the desirable qualities of the nozzle according to the present invention.